The present invention relates generally to a material handling apparatus and, more particularly, to an apparatus for handling sheet material.
It is common practice in the tempering of glass sheets to initially heat the sheets in a furnace to substantially the softening point of the glass and then to suddenly chill the heated sheets below the annealing range of glass to place the outer surfaces thereof under compression and the interior under tension. This process improves the mechanical properties of the glass by increasing the physical strength thereof and modifying its breaking characteristics so that when a tempered glass sheet is broken it will shatter or disintegrate into relatively small, harmless particles rather than dangerously large pieces having jagged edges. When forming bent tempered glass, a shaping operation is interposed between the heating and chilling steps to impart the desired curvature to the sheet.
One well-known expedient for handling glass sheets in a mass production operation is to support the sheet in a vertical plane when processing the same through the various stages of heating, bending and/or tempering. The glass sheets are supported vertically by means of tongs which grip the opposite faces of the glass sheet adjacent the upper edge thereof. These tongs may be suspended from overhead conveyors or elevators which advance the glass sheet through various stations in either a horizontal or vertical path to form the bent and/or tempered glass sheets. In processes wherein the suspended sheet is advanced vertically through the various stations, very often the chilling medium is a liquid bath into which the heat-softened sheet is ultimately lowered. Sometimes, however, the chilling medium may be cooling gases in the form of streams directed through a plurality of nozzles or tubes mounted in opposed blastheads against the opposite surfaces of the glass. In the latter procedure, it is important that provision be made for relative movement between the heat-softened glass and the blastheads to assure uniform tempering throughout and avoid an undesirable pattern of iridescent spots otherwise formed by the blast of the streams impinging against localized or concentrated areas of the glass sheet. The most expedient technique in providing this relative movement in a vertical line is to continuously advance the sheet between stationary blastheads. However, this poses a problem when glass supporting tongs are employed because the spacing of the blasthead tubes from the opposite surfaces of the glass sheet, which should be at a predetermined optimum distance in order to achieve a uniform and quality temper, must be compromised to provide sufficient clearance for the passage of such tongs between the blasthead tubes.